As discussed in Refs. [1]}, [2]} the four-quark local operator \(O_1\) 
admits a low-energy realization consistent with chiral symmetry and its breaking. To LO (and neglecting higher powers of the pion fields)
one has
\(O_1 \ \rightarrow \ g_1^{NN} \,\bar{p} n \bar{p} n\ + \ \frac{5}{6} g_1^{\pi \pi } \, F_\pi ^2 \, \partial _\mu \pi ^- \partial ^\mu \pi ^-\ + \ \sqrt{2} g_A g_1^{\pi {N}} F_\pi \, \bar{p} (S^\mu \partial _\mu \pi ^-) n + \ldots \) 
